A holographic technique is applied for digital watermarking by a computer. A digital-watermark image to be hidden is phase modulated in a random fashion, and its Fourier-transformed hologram is superposed on a content image. The watermark is reconstructed by means of a holographic-reconstruction technique from the bit-map image that hides it. In the study the processes of constructing and reconstructing a digital hologram are described on the basis of the theory of Fourier optics. The conditions for superposing the hologram onto the content images are investigated in detail. The validity of the present method is verified by changing the weighting of the hologram relative to that of the content image. The effect of image size is also discussed with respect to reconstruction of the watermark, and it is shown that watermark information in a form of a diffuse-type Fourier-transform hologram cannot be removed by cutting it out of the host image.
© 2002 Optical Society of America
(070.2580) Fourier optics and signal processing : Paraxial wave optics
(070.6020) Fourier optics and signal processing : Continuous optical signal processing
(090.1760) Holography : Computer holography
(100.2000) Image processing : Digital image processing
(100.3010) Image processing : Image reconstruction techniques
Nobukatsu Takai and Yuto Mifune, "Digital Watermarking by a Holographic Technique," Appl. Opt. 41, 865-873 (2002)